1. Field of the Invention
The present invention relates to devices for producing x-ray images by computer radiography, and in particularly to such devices having a storage plate and a reproduction means including an image reader for reading the photo-stimulated luminescence of the storage plate with at least one read-out beam on two sides of the plate.
2. Description of the Prior Art
In digital computer radiography, as is known, the image of the x-radiation after passing through an examination subject is retained in a storage plate and an image reader subsequently converts the image into digital electrical signals, so as to generate a visible image on a picture screen. In the storage plate, the x-rays are converted into electrical charge. For this purpose, the storage plate has a persistent storage luminophore which is cast in an organic binder and is applied to a transparent carrier, generally a foil. The image reader converts the planar pattern of various x-ray intensities into corresponding electrical signals, which are supplied to an image generating computer (image processor) via an analog-to-digital converter. After reconversion into analog signals, the image can be supplied to a display unit.
The storage plate is read by a high energy excitation beam, such as a red laser beam, and the photo-stimulated luminescence is successively supplied point-by-point to a common photomultiplier, and then to an amplifier via a light conductor. The laser beam is focused on the storage plate by suitable optics and is deflected or swept across the line by the action of a rotating mirror. The storage plate is shifted in steps relative to the fan of the laser beam, so that the entire image is read line-by-line by the laser beam. For double sided read-out, respective light conductors can be disposed at both flat sides of the storage plate, as described in U.S. Pat. No. 4,485,302.
In an apparatus for producing x-ray images, it is also known to use a substrate having storage (persistent) material on both flat sides. The substrate consists of a material which absorbs the low-energy component of the radiation. In such an arrangement, the various storage layers must be successively read-out, as described in European published application 0 112 469.
A good efficiency for the storage arrangement is obtained if a significant part of the x-ray quanta are absorbed by the persistent storage luminophore. For that purpose, the layer thickness of the storage luminophore should be in theory selected as large as possible. With increasing layer thickness, however, a loss in resolution occurs and the image become less sharp. The loss of resolution is caused by oblique irradiation by the x-rays at the edge of the storage plate. When the x-rays are incident on the storage plate at an angle relative to the normal, an excitation of the luminophore of the storage luminescence screen caused by that x-ray also occurs at an angle relative to the normal. Given an approximately punctiform x-ray, this results in an elliptical deformation. The distortion becomes greater at the edge of the storage plate as the thickness of the storage luminescence screen increases.
In the read-out of the storage plate with a very fine read beam, a decrease in the sharpness of the image also occurs due to smearing of the read-out regions caused by scatter of the read beam and of the excited light in the storage luminophore, which generally consists of polycrystalline material. Such scatter causes an effective diameter which deviates considerably from the actual diameter of the read beam. The deviation is approximately on the same order of magnitude as the layer thickness of the storage luminophore.
The layer thickness, therefore, cannot be selected arbitrarily large. The efficiency of the storage arrangement is thus correspondingly limited, so that the radiation dose must be increased in order to achieve adequate brightness in the read-out. Such a solution, however, results in an undesireable increase in the radiation load on the examination subject.
For investigating the influence of the radiographic image quality on the diagnostic precision, it is known to employ a double film cassette which contains a standard system and a low-dose system. Each system includes films and luminescent layers having respectively different properties. One film is disposed between two luminescent layers in each system. The two systems are separated by a lead foil. Two images with different radiation energy are thus obtained of the examination subject. These images are further processed for subtracting soft organs or bones so that these features can be made more visible. The noise is considerably higher and the low-dose system, and the image quality is considerably diminished. Such a system is described in Med. Phys. 11, Vol. 5, Sept/Oct., 1984, pages 646-652.